Several technologies have been used in the past to remove or annihilate organic contaminants found in hazardous chemical waters, wastewaters, and polluted gases. Some destructive techniques, e.g., chlorination, use strong oxidants that are themselves hazardous. On the other hand, the predominant non-destructive technologies currently in use have serious drawbacks: air stripping converts a liquid contamination problem into an air pollution problem, and carbon adsorption produces a hazardous solid which must be disposed. Thus, conventional methods for organic contaminant disposal must be replaced with procedures having minimal environmental impact.
Advanced oxidation processes (AOPs) are one example of an environmentally friendly approach for treating organic contaminants. AOPs usually involve treatment of the contaminant with ultraviolet light (UV), chemical oxidation, or both. AOPs are destructive processes in which the target organic compounds may be fully oxidized (i.e., mineralized) to relatively innocuous end products such as carbon dioxide, water, and inorganic salts. Because AOPs do not leave any residual contaminants requiring additional treatment, these processes are well suited for destruction of organic pollutants. Therefore, the development of effective AOPs is important.
Typical AOPs rely on the generation of hydroxyl radicals (OH.sup..cndot.) to degrade organic contaminants. The rapid, non-selective reactivity of OH.sup..cndot. radicals (one of the most reactive free radicals and strongest oxidants) allows them to act as initiators of oxidative degradation. Common AOPs such as H.sub.2 O.sub.2 /UV, O.sub.3 /UV, and H.sub.2 O.sub.2 /O.sub.3 /UV involve UV photolysis of O.sub.3, H.sub.2 O.sub.2, or both to generate OH.sup..cndot. radicals. In the photocatalytic oxidation, TiO.sub.2 /UV, a titanium dioxide semiconductor absorbs UV light and generates OH.sup..cndot. radicals mainly from adsorbed water or OH.sup.- ions. The overall process taking place in the photocatalytic mineralization of organic pollutants at a semiconductor (sc) surface can be summarized by the following reaction: ##STR1## where hv represents photons with an energy equal to or higher than the band gap energy of the semiconductor.
Semiconductor photocatalysis has been used to mineralize most types of organic compounds such as alkanes, alkenes, haloalkanes, haloalkenes, aromatics, alcohols, haloaromatics, haloalcohols, acids, polymers, surfactants, nitroaromatic, dyes, pesticides, and explosives. The susceptibility of such a wide variety of compounds to treatment in this fashion, makes photocatalytic degradation a particularly attractive process for air purification and wastewater treatment.
Under illumination, electrons (e.sup.-) and holes (h.sup.+) are usually generated in the space charge region of the semiconductor as shown in the following equation: EQU sc+hv.fwdarw.h.sup.+.sub.VB +e.sup.-.sub.CB (2)
Under proper conditions, the photoexcited electrons (in the conduction band, CB, of the semiconductor) and photoexcited holes (in the valence band, VB, of the semiconductor) can be made available for redox reactions. The photogenerated holes in the VB must be sufficiently positive to carry out the oxidation of adsorbed OH.sup.- ions or H.sub.2 O molecules to produce OH.sup..cndot. radicals (the oxidative agents in the degradation of organics) according to the following reactions: EQU h.sup.+.sub.VB +OH.sup.-.sub.ads .fwdarw.OH.sup..cndot..sub.ads(3) EQU h.sup.+.sub.VB +H.sub.2 O.sub.ads .fwdarw.OH.sup..cndot..sub.ads +H.sup.+.sub.ads (4)
The photogenerated electron usually reacts with oxygen according to the following reaction: EQU e.sup.-.sub.CB +O.sub.2 .fwdarw.O.sub.2 (5)
In most cases, the semiconductor can undergo oxidative decomposition by the photogenerated holes. It is generally found that only n-type semiconducting oxides are photostable towards photoanodic corrosion, although such oxides usually have band gaps which absorb only UV light. Thus, a desirable semiconductor suitable for reaction 1 will be: (i) photoactive; (ii) able to use visible and/or near UV; (iii) biological or chemical inert to agents to be treated; (iv) photostable; (v) inexpensive; and (vi) able to produce OH.sup..cndot. radicals, for example, as in, Eq. 3 & 4.
TiO.sub.2 and SrTiO.sub.3 satisfy the energy demand for reactions (3) or (4) and (5). Among the different semiconductors tested, TiO.sub.2 is the most efficient photocatalyst for reaction (1). TiO.sub.2 is effective not only in aqueous solution but also in non-aqueous solvents and in the gas phase. It is inexpensive, photostable, insoluble under most conditions, and non-toxic. Thus, TiO.sub.2 has proven to be the semiconductor of choice for photomineralization of organic pollutants.
In the photocatalytic oxidation of organics in aqueous solution, it has been shown that O.sub.2 reduction at the TiO.sub.2 surface is the rate determining step. This limitation can be overcome by the use of a porous TiO.sub.2 ceramic membrane, i.e., a "three phase" boundary system where the reactants are delivered to the reaction site as disclosed in co-pending U.S. patent application Ser. No. 08/791,599, filed Jan. 31, 1997, and incorporated by reference herein. However, enhancement of the photcatalytic reaction by assisting the oxidation/reduction reactions that take place on the TiO.sub.2 surface would be highly desirable.
Another disadvantage of conventional AOPs, such as O.sub.3 /UV and H.sub.2 O.sub.2 /UV, or their combination, is that they cannot utilize abundant solar light as the source of UV light because the required UV energy for the photolysis of the oxidizer is not available in the solar spectrum. Furthermore, some AOPs are efficient in mineralizing organic pollutants but exhibit slow kinetics, e.g., TiO.sub.2 /UV and H.sub.2 O.sub.2 /UV, while others exhibit much faster kinetics, but lower degree of mineralization, e.g., O.sub.3/ UV. Similarly, a limitation on the use of O.sub.3 in water treatments is the generation and mass transfer of sufficient O.sub.3 through the water to efficiently oxidize the organic contaminant.
Another type of AOP is the packed-bed photoreactor. Traditional designs of packed-bed photoreactors are based on the use of annular reactors filled with a photoreactive material such as glass particles coated with TiO.sub.2. The light source is placed either in the middle of the reactor (i.e., internal illumination) or outside of the reactor (i.e., external illumination). The internal illumination reactor is more compact than the external illumination reactor. However, light intensity per unit area decreases quickly with distance from the light source because of the absorption of light by TiO.sub.2 (Beer's law), but also because the farther from the light source, more active area needs to be illuminated. On the other hand, external illumination allows for the concentration of more light per unit area. However, the thickness of annular photoreactors is limited by the UV penetration due to its absorption by the TiO.sub.2 photocatalyst. Thus, in annular photoreactors, the amount of photocatalyst that can be packed is limited, i.e., only the length can be varied. Therefore, in order to achieve good convection and appropriate retention times, the annular photoreactor may require the use of long tubes and long UV lamps, and the length is limited by the pressure drop the reactor can sustain. These limitations, create a system that can be very bulky.
One problem with conventional photoreactors is that the amount of dissolved oxygen (or any other oxidant) in the water is very low compared to the amount of oxygen needed to photomineralize organic molecules to CO.sub.2 and water. Thus, conventional photoreactors can only be used for water polishing systems, for example, where the initial concentration of organic molecules in the water is approximately 1 ppm or less.
Therefore, there is a need for an improved AOP that provides efficient oxidation of organics in process water, contaminated ground water, polishing water systems, or polluted air. There is also a need for an AOP capable of utilizing UV light in the solar spectrum. In addition, there is a need for compact packed-bed reactors that are not limited by length and pressure requirements. It would be desirable if the process was cost effective, easy to operate, relatively fast, and capable of achieving total mineralization.